This invention relates to squirrel cage alternating current (AC) induction motors. More particularly, this invention relates to a process for securing a rotor cage to a rotor core in a rotor assembly, such as is found in a squirrel cage AC induction motor, and a rotor assembly formed therefrom.
The rotor assembly, such as is found in a squirrel cage AC induction motor, comprises a rotor cage, rotor core, and shaft. Typically, the rotor core comprises a series of disk shaped laminates stacked to form a hollow cylinder. The rotor core is most commonly steel but may be constructed of other materials. The core contains substantially longitudinal slots which may be surrounded completely by core material or may be open to the outer longitudinal surface of the core.
The rotor cage is also cylindrical in shape and is comprised of a plurality of rotor bars, mechanically and electrically secured by end rings. Rotor bars may be made of various material but most commonly are copper alloy or aluminum. The rotor bars are generally of comparable geometry to the slots. Numerous slot and rotor geometries have been successfully used in motor designs.
In a typical rotor assembly, the rotor cage is constructed by inserting the rotor bars in the rotor core slots, then securing the end rings to the bars by any method known in the art, for example brazing. Although components of the rotor cage are positioned within the rotor core, the cage is not typically fixed or secured to the core. This rotor assembly design results in some potential movement of the rotor cage with respect to the rotor core which is undesirable.
The comparable size of the slots and the rotor bars vary with temperature changes due to the differences in the expansion coefficients of the rotor bar and rotor core materials. Fluctuations in coefficients of the rotor bar and rotor core materials. Fluctuations in comparable sizes cause variations in the degree of movement between the rotor cage and the rotor core. Temperature variations may be caused by changes in current and speed during motor operation and by external temperature changes. These variations make it difficult to continuously maintain a close fit between the rotor cage and the rotor core. Absent a close fit, the rotor cage will move relative to the rotor core creating an imbalance. Resulting tilting of the rotor cage with respect to the rotor core may introduce unwanted axial forces on the rotor cage.
A cage positioned within the core at room temperature may shift when heated and settle in a different position when brought back to room temperature. Therefore, the relative position of the rotor cage and the rotor core at a particular temperature may vary, thereby affecting motor performance in an unpredictable manner.
Motor performance is also sensitive to rotor deviation from the central position of a surrounding stator. Movement of the rotor cage within the rotor core may contribute to unwanted rotor displacement within the stator, thereby reducing efficiency of the motor.
For the foregoing reasons there is a need for a method to reduce the movement between the rotor cage and the rotor core.